OK, on the assumption that you followed Russ' advice and reran the
relevant wizards, then the SBS should be set up correctly and you need
to find where the blockage is occurring.
Here is a useful technique for investigating SMTP servers:
First connect a computer temporarily to another LAN port on the router,
make sure you can ping the router and external SBS NIC, then try to
contact Exchange on the external IP address with this method. If you get
no reply, there is still an issue with the SBS configuration. If it is OK,
try again from outside your network, to test the router forwarding.
For a really quick and dirty test, go to http://grc.com from a LAN
workstation or your temporary router connection (do not surf with the
server) and select Shields Up!!, and click on the Common Ports button.
Ignore all the seriously alarmist messages, and just look for an open